A surface acoustic wave (SAW) sensor detects physical quantity such as pressure, load or acceleration by converting the physical quantity to a distortion of a SAW device. Specifically, the physical quantity is detected by detecting change in frequency, delay time, and phase of the SAW device.
Teshigahara, Akihiko; Hashimoto, Ken-ya; Akiyama, Morito “Scandium Aluminum Nitride: Highly Piezoelectric Thin Film for RF SAW Devices in multi GHz Range”, 2012 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IUS) describes a SAW device using SAW material having ScAlN/SiC structure. The SAW material has ScAlN film on the surface of SiC board. The ScAlN film is a piezoelectric thin film, and the SiC board is a non-piezoelectric board.
Since the piezoelectricity of ScAlN film is high, the SAW characteristic of SAW device having the ScAlN/SiC structure is excellent. Further, the SAW device can be used under high temperature environment since the heat resistance of ScAlN film is high. The SAW device having the ScAlN/SiC structure may be used in the surface acoustic wave sensor to realize a sensor with high sensitivity that can be used under high temperature environment.
In this case, however, an amount of change in frequency, delay time, and phase of SAW element is made smaller by the acoustoelastic effect when the SAW material is distorted by physical quantity, such that the detection sensitivity (sensing accuracy) of physical quantity may be lowered. The acoustoelastic effect is phenomenon in which the propagation speed of SAW in the SAW material is changed by stress (distortion).